Rotten Lemon, Waste Paper Help Recover Precious Metals

A Japanese university professor developed a technique to selectively recover precious metals from the fluid of melted electric/electronic parts by using adsorbent made from biomass waste.

The new technique can reduce the cost of recovery and environmental load because it can reclaims metals such as gold, silver, platinum and palladium by using, for example, used paper and rotten fruit. The technique was developed by Hidetaka Kawakita, an assistant professor at the Department of Chemistry and Applied Chemistry of the Faculty of Science and Engineering of Saga University.

There are several methods to recover precious metals from waste including solvent extraction process, which uses activated carbon, and other extraction processes that use polymeric resin adsorbent made of ion-exchange resin or chelating resin.

However, the adsorption capacity of these methods is as low as 1-3mol/kg. And, if a metal identification block is designed to selectively extract specific metal, the cost will increase to more than double. Furthermore, solvent extraction processes require a large scale wastewater treatment system because they use organic solvents such as toluene.

In regard to the extraction process based on polymeric resin adsorbent, the absorbed metals and the resin are separated from each other by burning the resin. This leaves tar, coke or other waste material, which requires cumbersome treatment, after the incineration.

In the new method, on the other hand, an adsorbent is made from biomass waste. Specifically, selected constituents are extracted from waste paper containing cellulose or lignin, or waste fruits, such as persimmon or lemon, containing a large amount of polyphenol. Then, those constituents are adjusted by an amination reaction. In this way, the new method can reduce the cost to 1/10-1/2 that of the existing methods.

In an experiment where a fruit-derived absorbent was used for the solution made of hydrogen tetrachloroaurate (III) hydrate melted by hydrochloric acid, the recovery rate of gold was 100%. Also, it was observed that a waste paper-derived absorbent selectively recovered platinum, palladium and rhodium depending on the kind of functional group used in the experiment.

The recovery rate of platinum and palladium reportedly exceeded 80%. The absorption capacity of the absorbents range from 3 to 10mol/kg, which is three times higher than that of activated carbon or polymeric resin. And the new method can reduce the environmental load because it does not use any harmful organic solvents.

This research was conducted as part of the Industrial Technology Research Grant Program promoted by Japan's New Energy and Industrial Technology Development Organization (NEDO).

Correction Notice: In the original article, we incorrectly stated, "In an experiment where a fruit-derived absorbent was used for the waste fluid made of electric/electronic parts melted by hydrochloric acid. ...," which is actually, "In an experiment where a fruit-derived absorbent was used for the solution made of hydrogen tetrachloroaurate (III) hydrate melted by hydrochloric acid. ..."